Telltale bacteria could reveal time of drowning

When a fisherman's body washed ashore on Australia's Queensland coast last week, police initially had no way of working out when he had died. "Unless a body is witnessed entering the water, there is no reliable method for determining the length of time that a body has been submerged," says Gemma Dickson, a forensic biologist at the University of Otago in Dunedin, New Zealand.

That could soon change. Dickson and colleagues have discovered how the type of marine bacteria colonising a body changes as it decomposes, providing a "clock" of how long bodies have been in the water.

At present, forensic scientists have no accurate way of estimating time of death for bodies fished out of water. Looking at how badly decomposed the body, for instance, is unreliable. And well-established methods that determine time of death for corpses on land, such as insect infestation, aren't likely to be useful with submerged bodies.

So Dickson and colleagues submerged three adult pigs' heads in Otago Harbour, New Zealand, while sampling the bacteria on their decomposing skin every two to four days. The heads were underwater for three weeks, or until they were reduced to a skull. To see how water temperature affected the bacteria, the team submerged the first head in autumn, the second in early winter and the third in late winter.

Dickson found that stages of decomposition had different bacterial signatures. For example, for the heads submerged in winter, Pseudoalteromonas bacteria colonised during the first stages of decomposition, while several genera of flavobacteria only emerged after 17 days underwater. The results will be published in the journal Forensic Science International.